People always say this but it isn't really true.
The literally thousands of past avatars did shit all except make the Avatar State stronger.
The nature of time means that each Avatar only really deals with problems that involve the past, maybe, half a dozen Avatars. Look at Aang, who was able to pretty easily access a bunch of them by the end of Book 3--Roku was the only one who was of real assistance, and Kyoshi when dealing with her specific incident involving the Conqueror. Having the other Avatars past those two did nothing for Yang except tell him he should probably just kill Ozai (advice he completely went against).
Whoever is the avatar after Korra is going to be dealing with the issues of the world Aang and Korra helped form. What the fuck are Ruko and Kyoshi going to contribute to a discussion about clashes between spirits and increasing technology, or the human settlement of the spirit world, or anything at all? They have zero context or experience with whatever the post Korra avatars are going to deal with. Past avatars are only good for helping clean up their own mistakes (Ruko on Ozai, Aang on Yakone, Kyoshi on Chen, etc) and making the state more overpowered.
Unless she is killed early, Korra will be a master bender in all four elements by the time she dies. She will be the second (third?) avatar to master spirit bending. She will be the first Avatar to know metal bending, she will be the only avatar in thousands of years with personal experience with Raava and Vaatu, the only one in thousands of years to know the complete story of the first Avatar, and the Avatar who existed during the rise of technology, the unification of the two worlds and the conflicts of the modern day.
The next avatar will be absolutely fine. The only downgrade is the power of the Avatar state, and it's extremely dangerous to hang out in constantly and will still be very powerful by the time they'd be using it. The metal alone makes her more valuable than most.